Core/shell structured latex particles with an interpenetrating polymer network (IPN) core (polymer A and polymer B) and glassy shell (polymer C) were synthesized by emulsion polymerization techniques. Polymer A was poly(butadiene-stat-styrene) (90/10) (P(Bd/S)). Polymer B was either poly(butyl acrylate-stat-methyl methacrylate) (60/40) or poly(butyl acrylate-stat-styrene) (60/40) prepared by a sequential IPN technique to form the cores. The glass transition temperatures, T-g, of polymer A and polymer B were around -70 and 10 degrees C, respectively. The shell, polymer C, poly(styrene-stat-acrylonitrile) (72/28), was polymerized with grafting onto the IPN core by a semicontinuous process. The particle size and particle size distribution were measured by light-scattering, capillary hydrodynamic fractionation, and transmission electron microscopy. The glass transition temperatures were determined by differential scanning calorimetry. The polymer A (particle sizes between 200-270 nm) with different gel fractions was used to characterize the effect of gel fraction on IPN morphology. A core/shell type phase separation was observed for IPN particles when using a polymer A with > 90% gel fraction. A distribution of polymer B domains in polymer A was obtained when using polymer A with a 40% gel fraction. A good coverage of polymer C on the IPN core particles was obtained at a core/shell ratio of one to one. A broad glass transition range for the core/shell structured latex particles was observed. In article II of this series, these latex particles will be used to toughen such engineering thermoplastics as polycarbonates to understand the particular toughening theories of these polymers.